Current-blockade particle-impact electrochemistry: A single-entity approach for digital (bio)sensing

Single-entity electrochemistry (SEE) represents a class of highly sensitive methods that employ miniaturized electrodes for the detection of individual analytes. Among SEE methods, particle-blockade impact electrochemistry studies the collision of non-electroactive particles to a micro or nanoelectrode surface, where individual collisions lead to step-like decreases in the current-time response. This thesis aims to study blockade impact electrochemistry, and to employ it as a single-entity method for the detection of single-strand DNA oligonucleotides. The motivation for this study is to design an electrochemical sensing method that can detect biomolecules in a digital on/off manner instead of the conventional ensemble response. Different aspects of blockade impact electrochemistry and particle-based sensing are discussed in the thesis; particle transport in different regimes in Chapters 2 and 3, electrode geometry and signal size distribution in Chapter 4, employing inner-sphere faradaic reaction in blockade impact in Chapter 5, and designing a competitive assay for particle-based detection in Chapter 6. Detecting at the single-entity level is the ultimate mass sensitivity that can be imagined for a sensing system. However, this improved sensitivity can be misleading when contemplating the use of these methods for the detection of analytes at ultralow concentrations. Employing a miniaturized transducer decreases the probability of interaction between the target and the electrode surface. Hence, despite the high mass sensitivity, micro and nanoscale elements cannot achieve biosensing at ultralow concentrations on a practical time scale. Chapter 7 investigates how parallelization can improve the concentration sensitivity in single-entity biosensors. It is argued that detecting biomolecules at ultralow concentrations can be achieved when miniaturized elements work as parallelized separately addressable single-entity sensors: digital electrochemical biosensors.